This invention relates to fluid compositions used in treating a subterranean formation. In particular, the invention is aqueous wellbore treatment compositions which are foams containing a viscosifying agent, a foam extender, a gas component, and a surfactant, as well as methods of forming such fluids, and uses thereof.
Various types of compositions are used in operations related to the development and completion of wells that penetrate subterranean formations, and to the production of gaseous and liquid hydrocarbons from natural reservoirs into such wells. These operations include perforating subterranean formations, fracturing subterranean formations, modifying the permeability of subterranean formations, or controlling the production of sand or water from subterranean formations. The compositions employed in these oilfield operations are commonly known as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, stimulation fluids, conformance or permeability control fluids, consolidation fluids, clean-out fluids, and the like.
These compositions often incorporate a gas component, such as air, nitrogen or carbon dioxide, to form a foam, energized fluid, or emulsion for treating the subterranean formations. It is desirable for these compositions to exhibit adequate viscosity and stability to perform the treatment, for example, to suspend and carry proppant into the fracture zone during a fracturing operation.
It is commonly known that stabilizing energized fluids or foams with viscosity properties suitable for oilfield operations become increasingly difficult to achieve at elevated formation temperatures, thus requiring higher levels of polymer or surfactant viscosifying agents. The matter is worsened when a gas such as carbon dioxide is present in the gas phase, since carbon dioxide exhibits high solubility in aqueous solutions. Subsequently, the carbon dioxide reacts with water to form carbonic acid, which may in turn reduce the effectiveness of metal crosslinking ions. Also, exposure to carbon dioxide at high temperatures promotes degradation of the polymeric chains, thus contributing to the referred loss of foam stability and viscosity. Further, it is commonly believed that the acidic effect of carbon dioxide cannot be overcome.
The viscosity of the compositions in which the gas component is dispersed may also affect the resulting viscosity and stability of the foam or energized fluid. In general, foams are more stable and viscous as the viscosity of the base fluid increases. Viscosifying agents such as viscoelastic surfactants and high molecular weight polymers are often added to increase the viscosity of the base fluid. However, a detriment of increasing the viscosifying agent content is a corresponding decrease in the retained conductivity of the formation after the treatment, as well as reduced clean-up, thus affecting well productivity negatively. Increased levels of viscosifying agent also lead to increased resource and material requirements.
The need to identify suitable chemicals to formulate viscous foams which provide improved retained conductivity, stability, and viscosity properties is known to those skilled in the art. A fluid that can achieve the above would be highly desirable, and these needs are met at least in part by the following invention.